Clonidine is a centrally-acting alpha-agonist used clinically to control hypertension. Acute or chronic treatment of spontaneously hypertensive rats with clonidine does not affect the apparent Km of tyrosine hydroxylase for the synthetic cofactor, 6-methyltetrahydropterine, when measured in vitro in hypothalamus, pons-medulla, forebrain, striatum or hippocampus.